It is believed that the use of communication systems in motor vehicles may be becoming increasingly widespread. The Society of Automotive Engineers (SAE) has defined three requirement classes relating to communications in motor vehicles. These classes may differ based on the priority of the messages to be exchanged, the varying real-time requirements, and the applications involved. Class C may be regarded as having demanding requirements. The SAE has published the following specifications regarding Class C: “Communication Protocols for Class C Applications”, SAE J2056/1, Jun. 1993. Class C may have particular significance in regards to future vehicle functions such as x-by-wire systems including, for example, steer-by-wire and brake-by-wire. These functions may be demanding in terms of real-time capability and reliable communication protocols.
Controller area network (CAN) bus systems (if necessary with upgrades for x-by-wire systems, and time triggered protocols for Class C (TTP/C)) may be used in these safety-critical vehicle functions. One of the key features of these protocols may be the global time base present for all stations of the bus system.
The global time base is a time base that may be valid throughout the communication system. It may play an important role in time control in communications (such as, for example, time control communication protocols, e.g., TTP/C) and in the application in question (such as time-controlled operating systems, e.g., Robert Bosch GmbH's XERCOS). It may also play an important role in diagnostic functions, error detection, and error management. If a global time base is used, this common time base may be present for all stations of the overall communication system, independently of what is responsible for generating the global time base. The local time base of a given station may be synchronized with the local time base of the bus system's other stations. This synchronization may be carried out based on the global time base.
The global time base constitutes an abstract notation, and the local clocks in the stations may only approximate to it. Thus, the stations have a local view of the global time base. The individual stations may only be connected to one another via the communication system. Because responsibility for the global time base may therefore only be assigned to the overall system, the communication system may bear this responsibility, for reasons of complexity.
The global time base may be generated according to two different models. In the first model, the global time base may be generated by a central master station of the bus system. This master may have a functionality that may be suitable for providing a global time base. The master may periodically send its view of the global time base to the other stations of the overall system. The other stations may take over the master's view of the global time base and make it their own global time base. In a communication system of this kind having a master station, synchronizing the global time base with an external reference time may not present any difficulties, provided the master station has access to the external reference time.
The situation may be different in the case of the second model for generating the global time base. The local views of the global time base may be exchanged among the bus system's stations via an algorithm that functions distributively in the communication system, e.g., a fault-tolerant average algorithm, and a difference may be calculated based on the values received. During a predefinable interval—the re-synchronization interval—the difference between the user's own view and the global time base may be corrected so that the view of a given station may be synchronized with the global time. In distributed bus systems of this kind, it may not be a straightforward matter to synchronize the global time base with an external reference time. Due to synchronization, an erroneous global time base may arise, i.e., the global time base may have substantial jumps or may be regressive.